A refrigerated transport container, or a so-called reefer container, is a shipping container used for freight transport. The container is cooled or refrigerated for the transportation of temperature-sensitive cargo. A typical reefer container consists of a refrigerated transport volume in connection with an active cooling unit that typically relies on external electric power supply.
The impact on society of refrigerated transport containers is vast, allowing consumers all over the world to enjoy fresh products at any time of the year and experience previously unavailable fresh products from different parts of the world. Most consumers take for granted that fresh products of all kinds are available and reasonably priced in every grocery shop all year round.
Providing consumers according to their expectations requires technologically advanced reefer containers with reliable, automated climate control systems and sophisticated logistics solutions.
The evaporator is the part of the refrigeration system in which the refrigerant absorbs heat from the transport volume and thereby cools air forced over or through the evaporator or evaporator coils by means of one or more evaporator fans. In this process, moist and water carried by the air passing over or through the evaporator may settle on the cold surfaces of the evaporator thereby initiating frost and/or ice build-up on the surface(s) of the evaporator. The frost and/or ice build-up accumulates over time to a level capable of hindering proper operation of the cooling unit if not countered by proper defrosting measures.
A defrost controller, or a defrost functionality incorporated to a processor controlling the operation of the cooling unit, therefore typically is provided.
In the following, for the sake of simplicity, the denomination “defrost controller” is referring to a defrost controller as such and/or the said defrost functionality incorporated to a processor controlling the operation of the cooling unit.
For a regular so-called defrosting cycle, or defrosting period, the defrost controller normally has two decision moments:
a) the first decision moment is when to initiate the defrosting cycle, and
b) the second decision moment is when to terminate the defrosting cycle.
The second decision moment may be set during a current defrosting cycle or prior to the first decision moment.
The first and the second decision moments may, according to prior art, be triggered e.g. by certain parameters reaching a predetermined threshold and/or the lapse of a given timeframe.
Frozen mode operation of a reefer container is operation at a temperature set point below, or well below, 0° C. or −5° C.
Three situations, which may occur during frozen mode operation, or in modes of operation wherein the return air temperature is about 0° C. and below, are:                i. the formation and build-up of frost and/or ice on the surfaces of the evaporator, and/or,        ii. the formation and build-up frost of and/or ice on components in the cooling space above or upstream the evaporator, and/or        iii. the formation and build-up of ice on one or more faces of the floor underneath the evaporator, i.e. in the supply air ducts.        
The first situation (i.) is considered a common situation. Prior art methods and systems for defrosting are configured for addressing this situation and the first and the second decision moments a) and b) as per above therefore are set or controlled to counter this situation.
The components housed or accommodated in the space above or upstream the evaporator and furthest away from the evaporator, as per situation (ii.) above, is the return air grid, which is arranged to separate the cooling space from the cargo space or transport volume without substantially hindering through flow of air. On some occasions, also the return air grid may be susceptible to formation and build-up of frost and/or ice, which in turn gives rise to air flow wise problems that, ultimately, need to be addressed in order to secure proper operation of the refrigerated container.
In case the return air grid blocks completely, air circulation in the container may be irreversibly blocked which in turn may necessitate repacking of cargo to another refrigerated container.
Formation of ice on components in the cooling space arranged above or upstream the evaporator, especially on the return air grid, usually occurs when the climate conditions in the refrigerated transport container include presence of super-cooled fog and/or ice crystals present in the return air flow.
Formation and build-up of frost and/or ice on or in the floor underneath the evaporator, as per situation (iii.) above, i.e. in the supply air duct, may also potentially irreversibly block air circulation in the container and necessitate repacking of cargo to another refrigerated container.
Repacking of temperature sensitive cargo is associated with temperature abuse, loss of time and the cause of substantial additional expenses to the shipping company.
Irreversible blocking of the air circulation can usually be avoided in frozen mode shipments, but incidentally does occur. In particular, irreversible blocking occurs in shipments with a very moist load, which load still needs to be cooled down after loading it into the container.
A conceivable solution to the problem of accumulating ice in the air ducts or elsewhere would be to simply supply heat to locations where ice may accumulate. Ice primarily tends to accumulate on the floor of the refrigerated transport container and on melt water collection guides provided on the inner walls of the cooling space. A solution to the problem of accumulating ice could be to avoid ice formation by supplying sufficient amounts of heat to the mentioned locations during defrost cycles. However, supply of heat to the mentioned locations would then require installation of heating elements such as trace heating elements in the location where ice is observed to accumulate to a problematic level, i.e. primarily in or close to the supply air duct region.
Since refrigerated transport containers need to be highly standardized due to requirements of the shipping industry in which they are used, it is unlikely that some containers could be specifically adapted for carrying very moist loads. Especially certain de facto shipping industry requirements, such as economy of scale, global utilisation and unlimited versatility of the container fleet, work against such adaptation of a minor portion of the reefer containers.
WO 2014/147076 A1 discloses a reefer container for shipping palletized cargo. The container includes a reefer machine where air circulating or passing through the reefer machine is divided into ducts at both sides of the container. The invention further relates to a method of loading a reefer container with pallets.
U.S. Pat. No. 3,465,534 A discloses an apparatus for defrosting the evaporator of a refrigerator when the flow of air through the evaporator becomes relatively obstructed by frost. The flow of air through the evaporator is measured with respect to the air flow in a reference path which bypasses the evaporator. The flow rates are sensed by a pair of self-heating thermistors, one thermistor being located in each of the paths. The thermistors are constructed of a semiconductor material having a transition temperature above which the resistance of the material rises abruptly. Thus, when current is applied to the thermistors to cause them to self-heat, the abruptly rising resistance characteristic causes the thermistors to be self-regulating at the transition temperature.
US 2003/202557 A discloses a transport temperature control unit and methods of defrosting an evaporator coil of a transport temperature control unit. The transport temperature control unit includes an evaporator coil, an ambient air temperature sensor for sensing an ambient air temperature, a return air temperature sensor for sensing a return air temperature, a discharge air temperature sensor for sensing a discharge air temperature, an evaporator coil temperature sensor for sensing an evaporator coil temperature, and a controller. The controller initiates a defrost cycle when a large temperature differential occurs over the evaporator.
US 2006/248904 A discloses a method of conditioning air in a vehicle load space. The method includes providing a refrigeration circuit including an evaporator, directing refrigerant through the refrigeration circuit, directing load space air across the evaporator, sensing a first condition based on one of a temperature and a pressure of the refrigerant in the refrigeration circuit upstream from the evaporator, determining a second condition based on one of a temperature and a pressure of the refrigerant in the evaporator, determining a difference between the first condition and the second condition, and initiating a defrost process of the evaporator when the difference is greater than a threshold.
U.S. Pat. No. 6,672,086 BB discloses a frosting cooler that on purpose supplies fog to the cooler. This is done with the object to create and maintain frost on cold products, such as bottles of a beverage stored in the cooler. By this is provided a visual manifestation of the cold condition of the beverage, which is meant to be highly attractive for thirsty consumers. The cooler has the ability to deliver moisture to the products within the cooler so that frosting may be produced in environments where there is low humidity in the ambient air without freezing the liquid contained by the bottle. The cooler is operated to control and to protect the frost on the products, once formed. In addition, the document discloses a design which prevents frost formation on objects in the air flow pathway between the product volume and the evaporator by just omitting or repositioning those components. Consequentially the mentioned specification does not describe a return air grid. Secondly it places the one or more evaporator fans such that air first hits the evaporator coil and then the fan, which is an effective way to avoid frost formation on the fan, though it leads to reduced refrigeration capacity and increased energy consumption.
Even if the frosting cooler described in the abovementioned U.S. Pat. No. 6,672,086 BB is designed and controlled to prevent frost build up on an evaporator and fan in an insulated cabinet in which products are to be stored, the environment is different as compared to a multi-purpose refrigerated transport container. As mentioned, a refrigerated container is a type of equipment that must be suited to carry any type of temperature-controlled cargo. The described frosting cooler also lacks a return air grid, which is an indispensable piece of equipment in a refrigerated transport container, through which return air passes on its way towards the fans and only then the evaporator coil.
In addition to the commercially motivated desire of utilising multi-purpose refrigerated transport containers without modifications, extra hardware comes with a purchase cost, requires installation, maintenance and occupies physical space needed for air flow etc.
Moreover, to position one or more evaporator fans such that air first passes the evaporator and then the evaporator fans, i.e. below the evaporator and as proposed by U.S. Pat. No. 6,672,086 BB, would supress the refrigeration unit's cooling capacity and energy efficiency. It would mean an energetic disadvantage in any shipment, to solve the problem of frost formation on evaporator fans in the rare scenarios where frost accumulates on the evaporator fans in the current design. As long as the evaporator fan position does not change, the return air grid needs to stay in place to avoid that objects, e.g. loose packaging materials or human fingers, hit the rotating evaporator fans.
In conclusion, there is a need for an efficient solution to above described problems, especially in frozen mode operation, or in modes of operation wherein the return air temperature is about 0° C. and below. The problems are pronounced also in situations where the temperature of moist loads is to be pulled down to a transport temperature below 0° C.
The primary problem to be solved by the present invention is to prevent formation and/or build-up of frost and/or ice on components in the cooling space above or upstream the evaporator, and in particular on the return air grid.
The secondary problem to be solved by the present invention is to prevent and/or counter the formation and/or build-up of ice underneath the evaporator, i.e. in the supply air duct(s) or in the entries of the T-bar flooring/grating on which the cargo rests.
Experience shows that the secondary problem is the consequence of not properly addressing the primary problem, i.e. as long as the return air grid is kept free of ice and/or frost, the abovementioned remaining parts of the refrigerated transport container will remain essentially free of ice and/or frost.